1. Field of the Invention
The present invention relates to a process for coating coating liquids on a continuously travelling web-like support (hereinafter referred to as a "web"), and more specifically, to an improved bar coating process.
2. Description of the Prior Art
Heretofore, various processes have been proposed to coat coating liquids on a continuously travelling web. Generally, it is considered that coating can be divided into a portion for transferring the coating liquid onto the web (hereinafter referred to as "system of application") and a portion for metering the coating liquid transferred to the web to the desired amount of coating (hereinafter referred to as "system of metering"), so that the coating process has been classified depending on the system of application and the system of metering. Known processes classified by the system of application are processes such as a roller coating process, a dip coating process and a fountain coating process, whereas known processes classified by the system of metering are processes such as an air knife coating process, a blade coating process, and a bar coating process. Further, known processes classified by both the systems of application and metering are processes such as an extrusion coating process, a bead coating process, and a curtain coating process.
Of these coating processes, the bar coating process, wherein excess coating liquid is transferred to the web and thereafter the surplus coating liquid is scraped by a bar which is fixed or rotated in the reverse direction as the direction of travel of the web at a peripheral speed lower than the web to obtain the desired amount of coating, has the characteristic that a thin coating at a high speed by means of a simple device and operation can be accomplished and as a result of this characteristic, the process has widely been used. While a suitable method may be used for the system of application in the bar coating process, the roller coating process, particularly, the kiss coating process, is the most commonly used method in an effort to make good use of the simplicity of the method. FIG. 1 schematically illustrates the bar coating apparatus, in which the kiss coating process is used as the system of application. Referring to FIG. 1, upon rotation of a roll 1, roll 1 picks up a coating liquid 3 in a liquid pan 2 and causes the coating liquid 3 to be transferred and coated on a continuously travelling web 4 to form a film 5, after which a bar 6, axially perpendicular to the direction of travel of the web, is brought into contact with the film 5 before the film 5 dries and solidifies to scrape off surplus of coating liquid 3, thereby metering to the desired amount of coating. Here, the bar 6 is formed by closely winding a wire having a given diameter around the surface of a rod (hereinafter referred to as a "wire bar") or having the surface of the rod formed with grooves of a given width and depth at a given separation, hereinafter pitch (hereinafter referred to as a "grooved bar"). Normally, the bar 6 is fixed, or intermittently rotated, or rotated in the counter direction to that of the web 4 at a peripheral speed lower than the web. The amount of coating may easily and accurately be controlled by suitably selecting the wire diameter of the wire bar, and the width, depth and pitch of the grooves of the grooved bar, and the like.
However, in the prior art bar coating process if a defect in the film surface is produced at the time of application such still remains as a defect even after metering by means of the bar 6. In case of the kiss coating process which is the most common system of application, coating stripes resulting from an uneveness in the thickness of a liquid film on the surface of the roll 1 considered to be caused by the flowing condition of the coating liquids within the liquid pan 2 often occur, and such a defect can not be eliminated sufficiently even by metering by means of the bar 6. In order to prevent the occurrence of such surface defects, proposals have been considered to additionally provide a metering and smoothing metal roll adjacent the roll 1 or to use a more precise application method. However, these approaches themselves result in a decrease of the most important advantage of the bar coating process, which is simplicity, and thus are undesirable.
In addition, in the prior art bar coating process, the system of application is completely independent of the system of metering, and therefore, in the process of coating, it is necessary to individually set the conditions, resulting in drawbacks, which are not only troublesome but require a large space making it uneconomical in space utilization.